1. Field of the Invention
The invention generally relates to white light emitting diodes, and more particularly to a white light emitting diode using phosphor excitation with the phosphor separated from the die.
2. The Prior Arts
One of the more common and mature white light emitting diode (LED) technologies is to coat or fill a yellow phosphor on or around a blue LED die. The yellow phosphor is excited by radiation from the blue LED and emits yellow lights. The blue lights emitted by the LED are then mixed with the complimentary yellow lights from the phosphor to generate two-wavelength white lights. However, with this technique, it is difficult to control the proportions of the participated blue and yellow lights and, as a result; the generated white lights usually have a non-uniform light color, too high a color temperature, and too low a color rendering index.
In another similar technology, red, green, and blue (RGB) phosphors are coated or filled around an ultra-violet (UV) LED. The phosphors are excited by the UV lights emitted from the UV LED, and the generated RGB lights are combined to form white lights. However, this technique still suffers the difficultly in controlling the proportions of RGB phosphors, although the UV lights themselves do not participate in forming the white lights. As a result, the color uniformity, color temperature, and color rendering problems are not satisfactorily resolved.
The foregoing techniques have another disadvantage. The phosphors coated on or filled around the LED die would be deteriorated due to the heat generated from the LED die itself, which would further compromise the performances of the LED including its color, brightness, and lifetime.
To solve the problem of uniformity, several solutions have already been disclosed. For example, U.S. Pat. Nos. 5,962,971 and 5,813,753 disclose that a filter was included in the LED package to improve the uniformity of the mixed white light. Taiwan Patent No. 569,479 discloses that a blue LED or an UV LED die was arranged in a fluorescent glue, and interposed between dielectric omni-directional reflectors so that the blue lights or UV lights are reflected repeatedly in all directions to excite the phosphors as much as possible, thereby consuming the energy of blue lights or UV lights and enhancing the white light conversion efficiency. However, the above-mentioned techniques could still not solve the phosphors' deterioration problem from direct contact with the LED die. Furthermore, the arrangement of the filter or the reflectors would also add to the process complexity, resulting in high manufacturing cost and low yield.
In view of the phosphor deterioration problem, Taiwan Patent No. M246,528 provides a white LED lamp, which can prevent the phosphors from heat deterioration by separating the phosphors from the LED die. However, this technique is designed for a lamp including many blue LEDs, but not for an individual LED.